The Vertical Shear Induced Secondary Circulation of Tropical Cyclones

The forced secondary circulation (FSC) by the vertical shear of horizontal winds is isolated from the latent heating and friction FSCs associated with a model-simulated hurricane vortex. This is achieved by use of a newly developed potential vorticity inversion and quasi-balanced vertical motion equations system. Results show that latent heating forces intense updrafts in the eyewall and slow subsidence in the eye, whereas the friction-FSC is similar to that of the Ekman pumping, with the peak ascent occurring near the top of the boundary layer in the eye. In contrast, a westerly mean shear produces an anticlockwise FSC across the vortex's core region with the rising motion downshear and easterly sheared horizontal flows. The horizontal sheared flows act to reduce the mean shear inside the storm, in the present case, by 25 – 30%, thus opposing the destructive roles of the environmental shear. When the mean shear is removed, the FSC becomes much shallower and reversed in sign due to the presence of negative shear in the eyewall interacting with the mean flow. Our findings appear to offer some new understanding of the impact of vertical shear on the dynamics of tropical cyclones and other types of cyclonic vortices.